The present invention relates to a method for dynamically balancing a rotating drum, such as a washing machine drum, by means of an automatic balancing device comprising at least one annular hollow body secured to said drum and a plurality of balancing bodies freely displaceable within said hollow body. During rotation of the rotating drum these balancing bodies move to compensate for unbalance in the system, by arranging themselves in positions counteracting the unbalance forces.
By such arrangements it is possible to achieve a quite effective balancing function when the rotating drum has reached its supercritical rotary speed, but during starting and acceleration sequences, i.e. at subcritical speeds, there still remains unbalance of such a magnitude, that it can be unacceptable for many applications.
This is particularly, but not exclusively, the case involving clothes washing machines, wherein such automatic balancing devices give a real balancing effect only during the spin-drying phase, but not during periods of normal washing.
This problem is the basis for EP-A-0 810 316 which describes a clothes washing machine with an outer casing and a suspended oscillating washing assembly comprising: a wash tub, a cylindrically shaped perforated drum housed in said tub and capable of rotating in the interior thereof about the axis of the cylinder during washing and spin-extraction, a motor capable of rotatably driving said drum at various speeds and provided with accessory means that are firmly associated with said tub, said drum being provided with a dynamic balancing arrangement formed by a plurality of annular hollow bodies with preferably a rectangular shaped cross-section, said bodies forming sealed closed-loop configurations and being mounted integrally with said cylindrical drum with their axes substantially coinciding with the rotational axis of said perforated drum, and a plurality of moving masses capable of freely moving in a liquid of a definite viscosity filled in said hollow bodies, and which is characterized in that before at least a spin-drying phase, the drum is driven to rotate in a continuous manner at a variable, relatively low speed in a single direction of rotation, wherein the frequency of said rotation is lower than the frequency of resonance of said oscillating washing assembly, but anyway sufficient in view of causing the washload in the drum to keep adhering against the inner peripheral surface of the drum.
Due to these features it is alleged that the balancing device is capable of effecting balancing during all the rotational phases undergone by the rotary drum with its washload contained therein.
The solution presented in EP-A-0 810 316, the disclosure of which being incorporated herein by reference, only takes into account the forces caused by the rotational speed variations, and it has been found that this is only correct under specific circumstances, which means that further conditions must be satisfied for ascertaining that balancing effect is obtained over all speed ranges every time.